Which of the following is a characteristic effect of antihistamines?

Antihistamines are primarily used to treat allergic reactions and conditions such as hay fever, hives, and some cold symptoms. A well-known characteristic effect of many first-generation antihistamines is sedation, which leads to drowsiness. This sedative effect occurs because these medications can cross the blood-brain barrier and interfere with the action of histamine, a neurotransmitter that promotes wakefulness and alertness.

Drowsiness can vary among different antihistamines, with first-generation antihistamines, such as diphenhydramine and chlorpheniramine, causing more sedation compared to second-generation antihistamines, like loratadine and cetirizine. However, drowsiness remains a recognized side effect of many antihistamines regardless of generation.

The other options do not align with the common effects of antihistamines. Increased alertness and heightened energy levels are usually associated with stimulants, which serve a different purpose. Meanwhile, increased anxiety is not typically a noted effect of antihistamines; rather, these medications are often used to alleviate symptoms associated with allergies, which can contribute to discomfort or anxiety related to those symptoms.